The present invention relates to a combustion air regulating apparatus for use with induced draft furnaces, and is directed more particularly to a combustion air regulating apparatus which is suitable for use with single and multi stage induced draft furnaces that include inducer motors which have one or more non-adjustable operating speeds.
In the operation of an induced-draft gas-fired furnace, combustion efficiency can be optimized by maintaining the proper ratio of the gas input rate and the combustion air flow rate. Generally, the ideal ratio is offset somewhat for safety purposes by providing for slightly more combustion air (i.e., excess air) than that required for optimum combustion efficiency conditions. In order that furnace heat losses are minimized, it is important that this excess air level is controlled.
In practice, the rate of combustion excess air flow is affected by a number of factors including vent length, furnace size, and wind conditions. Although furnace size may be predetermined at the factory, vent length is commonly not known until actual installation time, and wind conditions are normally highly variable during operation of the furnace. Additional conditions such as partial blockages by debris of various kinds can also affect combustion excess air flow rate while the furnace is in operation.
In addition, a large number of different furnace models are commonly in use at present, and it is highly desirable to provide an apparatus which can be used with a variety of different furnace models currently in use, as well as those that may be manufactured in the fixture. More specifically, it is desired to have a way of controlling excess air in single and multi stage furnaces of both the condensing and non-condensing types.
One method for controlling excess air in 2 stage induced draft furnaces having a fixed-firing rate is described in U.S. Pat. No. 4,729,207 (Dempsey et al). This patent teaches a method of air combustion regulation which makes use of a variable speed electronically commutated motor or ECM that is used in conjunction with a physically separate electronic control network. A method of combustion air regulation which makes use of a variable speed inducer motor having an integrated or built-in, software based electronic controller is described in U.S. Pat. No. 5,331,944 (Kujawa et al). Both of the latter patents are assigned to the assignee of the present invention, and both are incorporated by reference herein.
Another way of controlling excess air in induced draft furnace is to provide an air/fuel ratio control system. In systems of this type the rate of flow of combustion air and the rate of flow of fuel are so coupled to one another that changes in one rate of flow cause corresponding changes in the other rate of flow, thereby maintaining a constant percentage of excess air. In one system of this type, called a fuel primary system, the input controller controls the rate of fuel flow and the rate of flow of air is adjusted as necessary to maintain the desired excess air level, usually by means of butterfly valves that are controlled by transmitter-controller devices. In another system of this type, called an air primary system, the input controller controls the rate of air flow and an air/fuel rates controller causes the rate of fuel flow to be adjusted as necessary to maintain the desired excess air level. Systems of these types are described in greater detail in the "North American Combustion Handbook", third edition, volume II, pages 46-60.
While the safety aspect of excess air control is equally significant in residential and commercial heating applications, initial cost and fuel efficiency considerations often call for the use of different excess air control system for these different applications. This is because the fuel savings resulting from accurate excess air control in commercial heating applications are so much greater than those resulting from accurate excess air control in residential applications. As a result, furnaces that heat commercial buildings usually use excess air control systems of the above-described air/fuel ratio control type; and justify their higher initial cost on the basis of their greater efficiency. Some furnaces that heat homes, on the other hand, have usually controlled excess air levels by using one of the above-described types of variable speed inducer motor control systems. While the cost of these variable speed inducer motor control systems is less than that of air/fuel ratio control systems, that cost is still substantial.
Prior to the present invention, attempts to reduce the cost of excess air control systems of the variable speed motor type by replacing those inducer motors and their associated speed controllers with inexpensive motors having one or a small number of non-adjustable speeds have not been very successful at controlling excess air. One reason is that such systems still require controllers to select among the various non-adjustable speeds. Another is the fact that, particularly at their lower speeds, such motors have poor low speed operating characteristics. This in turn, caused the excess air level to fluctuate to an unacceptable degree as a result of variations in operating conditions, such as line voltage, wind speed and installed vent length.